Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an elect rode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
The factors to be considered in choosing a current collector for an ultracapacitor include electrical conductivity, chemical stability to electrolyte and contaminants and low electrical resistance throughout the life of the device. A current collector has two components of resistance; a material electronic resistance and a contact resistance at the electrode interface. Since the contact resistance can be significantly larger than the material electronic resistance, treatments to reduce contact resistance or enlarge contact area between the current collector and electrode are important. Surface etching and other roughening procedures can be used to enlarge contact area. However, the permanence of a treated current collector is an issue in device longevity since the surface of a collector can become chemically transformed, as with an oxide or can react with electrode or electrolyte to form a barrier layer.
Several types of materials are commonly used as current collectors in double layer ultracapacitors. Precious metals, such as platinum and gold are suitable although gold can be oxidized under certain conditions. Cost considerations exclude the precious metals from development. Conductive polymer materials such as polyethylene and Kapton.RTM. films loaded with 20-40 weight % carbon black show high resistance relative to metal films, but can be used if current flow is along the z axis, or thickness, of the film. However for current collection and conduction out of one or more cells in series, the polymer materials are unsuitable due to high resistance in the x-y plane. Additionally, conductive polyethylene and conductive Kapton.RTM. are slightly permeable to electrolyte, which diffuses through the polymer current collector with time. This effect is aggravated over time by high cell voltage and high temperature and is especially prominent on a cathode side of a cell. Further, Kapton.RTM. swells in common propylene carbonate ("PC") and .gamma.-butyrolactone ("GBL") electrolyte solvents and may cause other mechanical defects.
Current collectors commonly are made of aluminum because of its conductivity and cost. However, contact resistance is a critical factor with aluminum. Aluminum contact resistance arises from aluminum's superb ability to form a highly insulating oxide coating. Initially, a native oxide coating is easily breached by mechanical penetration of carbon particles during cell fabrication. Thus, aluminum collectors show low resistance when an ultracapacitor is first sealed. However with time and applied voltage, resistance can increase by an order of magnitude or more. This rise is caused by increased contact resistance and prohibits all but very low power operation. A mechanism postulated for this resistance increase is that mechanical action of the carbon particles at the aluminum interface breach the oxide coating and allow fresh aluminum to be oxidized beneath the particles. This results in a thickening of the oxide under each carbonlaluminum interface.
The present invention relates to a conductive and adherent coating for aluminum that prevents formation and thickening of the highly resistive aluminum oxide layer in a nonaqueous ultracapacitor.